The present application relates to a solid electrolyte cell and a positive electrode active material, more particularly to a solid electrolyte cell having a solid electrolyte not containing any organic electrolyte solution and to a positive electrode active material for use in the solid electrolyte cell.
Lithium ion secondary cells and batteries utilizing doping and dedoping of lithium ions are widely used in portable type electronic apparatuses and the like, because of their excellent energy density. Among the lithium ion secondary cells and batteries, totally solid lithium ion secondary cells and batteries using a solid electrolyte not containing an organic electrolyte solution as an electrolyte are under energetic research and development, from the viewpoint of safety and reliability.
As a form of the totally solid lithium ion secondary cell, a thin film lithium secondary cell is under vigorous development. In the thin film lithium secondary cell, the cell components such as current collectors, active materials and an electrolyte are each composed of a thin film to constitute the secondary cell. The thin films constituting the thin film lithium secondary cell are each formed by such a film forming method as sputtering, vapor deposition, and so on (see, for example, Thin-film lithium and lithium-ion batteries, J. B. Bates et al.: Solid State Ionics, 135, 33 (2000)).
In the thin film lithium secondary cell, an amorphous material such as LiPON, obtained or as if obtained by subjecting Li3PO4 to substitution by nitrogen, and LiBON, obtained or as if obtained by subjecting LixB2O3 to substitution by nitrogen, is used as a solid electrolyte. The amorphous material has an ionic conductivity of about 10−6 S/cm, which is a very low value as compared with the ionic conductivity level of normal liquid electrolytes of 10−2 S/cm. Since the film thickness of the solid electrolyte is small (e.g., about 1 μm) and the Li traveling distance is short in the thin film lithium secondary cell, however, the solid electrolyte composed of the amorphous material having the low ionic conductivity can exhibit substantially the same level of performance as those of liquid electrolytes.
On the other hand, in the thin film lithium secondary cell, the component having a rate-determining effect on electrical conduction is the positive electrode active material. In the thin film lithium secondary cell, in general, a lithium-transition metal oxide such as LiCoO2, LiMn2O4, LiFePO4, etc. is used as the positive electrode active material, like in liquid lithium ion secondary cells. In addition to these related art lithium-transition metal oxides, new lithium-transition metal oxides for use as the positive electrode active material have also been proposed. For instance, Japanese Patent No. 3965657 proposes crystalline LiCu1+xPO4 as a lithium-transition metal oxide for use as the positive electrode active material. These lithium-transition metal oxides (hereinafter referred to as the “above-mentioned lithium-transition metal oxide(s)”) are low in ionic conductivity and electronic conductivity.
In the thin film lithium secondary cell, the thickness of the positive electrode active material layer is proportional to the cell capacity. In order to obtain a high capacity, therefore, the positive electrode active material layer is preferably as thick as possible. In the thin film lithium secondary cell, however, thickening of the positive electrode active material layer formed of a material which is low in ionic conductivity and electronic conductivity (for example, to a thickness of 10 μm or more) leads to a very high internal impedance.
Accordingly, it is difficult to put to practical use a high-capacity thin film lithium secondary cell in which the above-mentioned lithium-transition metal oxide low in ionic conductivity and electronic conductivity is used as the positive electrode active material and the thickness of the positive electrode active material layer is enlarged.
Besides, the above-mentioned lithium-transition metal oxides are normally used in a crystalline phase. In the thin film lithium secondary cell, therefore, in forming a film of the above-mentioned lithium-transition metal oxide, a crystalline phase is formed by heating of a substrate during film formation, or by post-annealing after film formation, or the like.